Vacuum-tight windows



United States Patent VACUUM-TIGHT WINDOWS Thomas P. Vogl, Penn Township, Allegheny County,

Ralph O. McIntosh, Forest Hills Born, and Max Garbuny, Penn Township, Allegheny County, Pa., assignors to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Filed Mar. 26, 1956, Ser. No. 574,043

9 Claims. (Cl. 250-213) This invention relates to the manufacture of vacuumtight seals between a metallic member and a non-metallic member.

The principles of this invention may be embodied in a structure such as described in copending application 304,502, filed August 15, 1952, by M. Garbuny and J. S. Talbot, entitled Photothermionic Image Converter, and assigned to the same assignee.

The device as described in the above-mentioned application is what may be referred to as a thermal image tube. In the design of the photothermionic image tube, one of the requirements is a suitable input Window capable of transmitting both the thermal radiations of 8 to 12 microns as well as a visible scanning light beam with a minimum of optical distortion. It is necessary that both the thermal scene and the visible light source be imaged on a photoemissive surface which by its inherent nature must be maintained in a vacuum of pressure less than 10- millimeters of mercury. It is found that there are only a few materials that are capable of transmission of wavelengths within these two regions. It is also found that it is not possible to obtain a suitable window frame material having a similar coefiicient of thermal expansion as suitable window materials to maintain close fits to obtain good vacuum tight seals.

, It is, therefore, an object of this invention to provide an improved type of vacuum-tightseal.

It is another object to provide a frame structure for sealing a window thereto having a different coefficient of expansion than the frame.

It is another object to provide an improved method of sealing to an envelope a window capable of transmis sion of visible and thermal radiations.

It is another object to provide an improved method of selection of suitable windows capable of withstanding temperature changes during the sealing process.

It is another object to provide an improved thermal image tube.

.These and other objects are effected by our invention as will be apparent from the following description taken in accordance with the accompanying drawing throughout which like reference characters indicate like parts, in which:

' Figure 1 is a cross-sectional view of a thermal image device incorporating our invention;

Fig. 2 is an enlarged sectional view of the input window shown in Fig. 1; and

Fig. 3 is an enlarged, partial view of the sealing ring and window of the image tube shown in Fig. 1.

Referring in detail to Fig. 1, a vacuum-tight envelope is shown comprised of a glass cylinder portion 11 having Kovar sleeves l2 and 14 fused to the cylinder at opposite ends. One end of the cylinder 11 is sealed off by means of a reentrant portion 13 sealed to the Kovar sleeve 12, while the other end of the cylinder 11 is sealed oh by means of an input window 20 connected by means to be described to the Kovar flange l4.

' The input window 20 is ofsuitabletransmissive material such as barium fluoride having a diameter of the order of 3 inches and is sealed to a cylindrical member 22 by means of filler material 24 (Fig. 3). The member 22 may be of a noble metal or have an inner surface of noble metal with suitable coefficient of expansion. In the specific embodiment, a fine silver member 22 made by a suitable method such as spinning is in turn brazed on the opposite end to a Kovar flange 28 by means of an intermediate steel adapter ring 26 (Fig. 2). The Kovar flange 28 may be welded to Kovar flange 14 to seal the end of the cylinder 11.

Positioned on the reentrant portion 13 which closes off one end of the cylinder 11 is a heat conductive block 30 such as copper which is cooled on the exterior surface by means of a cooling loop 32. Positioned on the inner surface of copper block 30 is a heat sensitive screen or retina 34 which consists of essentially a layer 36 of photoemissive material, such as cesium antimony, and a layer 38 of infrared absorbing material, such as gold black, supported on the copper block 30. The layer 38 is sandwiched between the copper block 30' and the layer 36. Also provided within the envelope 10 is a collector electrode 40 for receiving electrons either directly or indirectly from the photoemissive surface 36 of the retina 34. The collector electrode 40 is provided with alead to the exterior of the envelope 10 for connection through an amplifier to the grid of a display tube.

Means are also provided for focusing the thermal image onto the retina 34 through the input window 20 and this focusing means may be in the form of a Cassegrainian telescope collecting parabolic mirror 44 and hyperbola mirror 46. The mirror 46 is positioned with respect to the collecting mirror 44 to reflect the thermal radiation which is collected by the parabolic mirror 44 onto the photoemissive layer 36 of the retina 34.

On the opposite side of the hyperboloid mirror 46 with respect to the input window, there is provided an auxiliary scanning light source 50 which may be a conventional kinescope. A lens 52 may be provided, as well as a filter 54, for focusing and directing light of desired wavelength from the light source 50 onto the photoemissive layer 36 of the retina 34.

reflector 44 and the mirror '46. The thermal image impinging on the retina 34 beats certain elemental areas of the photoemissive layer 36, thereby forming a temperature image corresponding to the temperature pattern of the observed scene. The elemental areas of the retina 34, which have been heated, now have electrons With higher average kinetic energy than the average energies of the electrons in the unheated areas of the photoemissive surface 36. This thermal image formed on the retina 34 may be scanned by a light beam from the kinescope 50 which causes the electrons to be emitted from the photoemissive layer 36 with a density that is a function of the temperature of the retina 34. The higher the temperature of an elemental area due to the thermal radiation thereon, the greater will be the density of the electrons emitted from the photoemissive layer 36. It may also be desirable in some applications to use materials that give less emission with higher temperatures. As the light beam from the kinescope 50 scans the retina 34, the electrons emitted will impinge on the collector 1 electrode 40 producing a current pulse in an output cirgrid of a conventional display device. such as a kinescope,

to obtain a light image corresponding to the thermal image on the retina 34. A more detailed description, of

the operation and structure of such a thermal imaging device may be found in the above-mentioned application.

Referring in detail to Figs. 2-and 3 of the drawing, there is shown the detail structure of the input window 20. In the design of a suitable window for thermal image devices, the only material known capable of transmitting medium wave thermal radiations, as well as-capable of being sealed vacuum-tight and withstanding high temperatures is silver chloride. This material, though it may be sealed to provide a vacuum-tight windownsuffers from several drawbacks due to the inherent physical properties of the material. It is a soft ductile material and, therefore, will deform under atmospheric pressure. It is also sensitive to actinic radiations and, therefore, cannot be exposed to wavelengths shorter than 4500 angstroms. It is also chemically very unstable and corrosive to most metals, as well as susceptible to attack by cesium vapor. It was, therefore, found that a more suitable material was barium fluoride which has many advantages over silver chloride. It will transmit all radiation from 1800 angstroms to 12% microns and is not affected by actinic radiation. It is also glass-like and literally crystal clear in appearance and texture, and may be polished to an optical finish with a minimum of difficulty. It has also a very low index of refraction which reduces materially the shift of oblique rays and reflective losses.

The window material which includes barium fluoride as supplied by the producer, consistsof blanks which are diamond saw cut to the thickness desired, as well as cut to the diameter desired. It is found that the growing process of such blanks introduces strains into the blank material and in order to prevent thermal shock cracking of the windows during the sealing process, it is necessary to select the blanks by inspection between crossed polarizers by techniques well known in the art of photoelastic strain analysis. A detailed description of this type of analysis may be found in an article entitled Photoelastic Strain Analysis Useful in the Design of Metal Parts" by M. M. Leven and published in Methods and Materials in the March and April issues of 1951. By techniques set forth in the above-mentioned article, the blanks may be examined in the polarized light, and interference effects are observed which can be related to the state of strains within the blank. When using monochromatic polarized light, a configuration of light and dark bands, known as fringes, will be observed in the blank. This configuration of fringes is referred to as a fringe or strain pattern. By the simple process of counting fringes and multiplying by a calibration constant known as a fringe value, the stresses throughout the blank can be calculated and expressed in pounds per square inch. By thus determining the residual stress in pounds per square inch within a blank, it is possible to ascertain the rate of heat which can safely be utilized in the sealing process of the window, as will be described. Thus, it has been found that the heating and cooling cycle in sealing a windowcan be matched exactly to the requirements of the particular blank. Furthermore, those blanks which show excessive strain can be discarded at the outset and thus assure that only satisfactory blanksacetone and alcohol'to remove all traces of grease and oil, the window is ready for insertion into a form or frame in which it is to be'sealed.

The frame assembly. into.- which; the. blank, is in erted.

consists essentially of three parts. These three parts are the frame 22, the transition ring 26 and a suitably shaped member for attaching the window to the tube body, such as the glass cylinder 11. In this particular application, a spun Kovar flange 28 was used for this purpose. Stainless steel, nickel or any other suitable material, which may be joined to the transition ring 26, may also be employed.

The transition ring 26 is of a material such as cold rolled steel. The transition ring 26 serves two functions: to permit copper brazing of the Kovar flange 28 to one side, and eutectic silver-copper brazing of the silver frame 22 to the other side. It also acts as a transition member with a coefiicient of thermal expansion intermediate between that of the Kovar flange 28' and silver frame 22.

The frame 22 is made of a thin sheet of a suitable material such as silver in order to minimize stresses applied to the rim of the barium fluoride blank or window 20. The frame 22 should'be in the form of a sleeve that is longrelative to the thickness in order to provide a flexible member between the blank 20 and the transition ring 26. Silver having-a larger coefficient of thermal expansion than barium fluoride, upon cooling places the latter under compression, a stress under which barium fluoride is very strong.

The configuration of silver frame 22 about the win-v dow 20 permits a vacuum-tight seal. Referring in' detail to Figs. 2 and 3', the silver frame 22 should fit the window 20 about its periphery with only afew mils clear ance for about /2 of the thickness of the window 20. This small clearance fit region as indicated by numeral 60' on Fig. 3 is the lower portion. of the window 20. The remaining part ofthe silver frame 22 above the small clearance fit is of larger diameter with abrupt change in clearance permitted. This larger clearance region is indicated by the numeral162 on Fig. 3. The larger clearance region 62 provides for the introduction of a filler material of suitable plasticity, melting range and wetting properties. One suitable material is silver chloride. Silver chloride used in this type of application is not effected detrimentally by actinic radiation. Below the window 20 and the region 60, the silver frame has an upwardly directed inturned' portion or rib 64. The rib 64 provides a shoulder 68 of slightly. smaller diameter than the window 20 and therefore supports the window when first placed in the silver frame 22. It is desirable that the shoulder 68 be as close as possible to the periphery of the window after sealing. Since the window will not expand as much, as the frame during the heating cycle, it is necessary to provide sufli'cient widthto insure support of the window. during heating. Theupturned rib provides a small channel regionr66ibetween the region 60 and the shoulder. 68. It is important that the plane of the shoulder 68 be perpendicular to the axis of the region 60,- and that when the assembly is mounted inthe sealing furnace. theplane of the shoulder. 68 be horizontal.

The window frame assembly consisting of the'frame 22, transition ring 26 and Kovar flange 28, is also degreased and the window20 isinsertedv into the frame 22 and seated on shoulder 68 for the sealing process. A strip of silver chloride filler, which is about .02 inch in thickness and of such a width that :when inserted edgewiseinto the region 62 it occupies approximately half the heightof region 62, is inserted betweenthe window 20 and'the frame 22;

The volume of the strip shOuld'be' slightly larger than the volume. of the region 60.

The assembly is then placed in a controlled atmosphere furnace with the shoulder 68 horizontal. An inert atmosphere, such as argon, is then introduced, and the temperature of; the furnace is increased. For a typical 3% inch diameter barium fluoride blank of a thicknesszof A. inch'flnia residual, stressof 800 pounds per square inch, a heating and cooling rate between 1' to 15 C. per minute is satisfactory. A higher rate might tend to crack the window by thermal shock. It is also found that thinner windows may be heated more rapidly. The heat is applied until the silver frame 22 reaches a temperature of 460 to 465 C., that is, 5 to C. above the melting point of silver chloride, and itis held there for approximately minutes. As the temperature reaches the melting'point of silver chloride, the silver chloride will melt and wet the frame and the window. The region 60 will be of a greater width than when two members are cold due to difference in coeflicients of thermal expansion but still small enough to permit capillary action. The close clearance of region 60 will allow the silver chloride to fill the space between window and silver frame 22. The excess silver chloride filler will also flow into the channel region 66. A vacuum-tight seal will be formed between the shoulder 68 and the window 22. Further, when the assembly is cooled, a compressive force will be placed on the window 20 which will be cushioned by the silver chloride remaining in the region 60. All this occurs both by gravity and capillary action. Independent of the coeff cients of thermal expansion, the window 20 always rests on the shoulder 68, providing an intimate contact to promote capillary action. The temperature of the furnace is then lowered at the same rate as it was increased. The window 20 and frame assembly may then be removed from the furnace and a vacuum-tight window including in whole or in part barium fluoride is obtained.

It is important to provide the cushion layer of filler material in the region 60. If only suflicient fillers were used to make the vacuum-seal on the shoulder 68, the window 20 would be subjected to a shear stress. Shear stress can be resolved into a compressive component and a tensile component. The window materials having suitable optical properties are much stronger in compression than in tension. During cooling after filler has become frozen, the filler pad provided around the periphery of window 20 within the region 60 exerts an outward force on the window frame 22. This force is transmitted to shoulder 68 and counteracts the contraction of the frame 22 on further cooling. It is therefore seen by placing the shoulder region 68 very close to periphery of window 20, the shear forces resulting from differential expansion are small.

It is found that seals which are formed by capillary action are less susceptible to voids, inclusions and other defects which may cause leaks than seals made where clearance is greater than that required for capillary action. It should also be noted that the shoulder region 68 serves as a confining passageway for filler materials to keep the filter from draining out of region 60.

While we have shown our invention in only one form, it will be obvious to those skilled in the art that it is not so limited, but is susceptible of various other changes and modifications without departing from the spirit and scope thereof.

We claim as our invention:

1. A vacuum-tight window for a thermal imaging or detection device comprising a transmissive window portion having an inner face and an outer face, said window including barium fluoride sealed to a flexible metal frame of a noble metal, said frame having an internal rib, said rib turned inwardly and upwardly to provide a shoulder of slightly smaller diameter than said window portion on which said inner face of said transmissive window portion rests.

2. A thermal imaging device comprising an envelope of material opaque to thermal radiations, a photorensitive electrode positioned within sad envelope, sa d envelope having an opening therein, means for directing thermal radiations from an observed scene onto said photosensitive electrode, an annular metallic flange member sealed to said envelope around said opening, a window frame assembly sealed to said flange member, said window frame assembly comprising a transmissive window including barium fluoride, a matching flange memher, a frame member of a noble material and a transition ring positioned between and sealed to said matching flange member and said frame member, said matching flange member sealed to said annular flange member, said transmissive window sealed within said frame member by means of a filler material of given plasticity.

3. A vacuum-tight window structure comprising a transmissive window portion of a nonmetallic material. sealed to a metallic frame member, said window portion having an inner face, an outer face and an edge portion, said frame member having an internal rib adjacent said inner face of said window portion, said rib turned upwardly to provide a shoulder of smaller diameter than said window portion to support said window portion, a cushion of filler material having a composition differing from that of said window portion disposed within the region between said edge portion of saidwindow and said frame member and a layer of like filler material between said rib and said inner face of said window, said inner face of said window being disposed with relation to said rib portion to promote capillary action.

4. A vacuum-tight window structure comprising a transmissive window portion of a nonmetallic material sealed to a metallic frame member, said window having an inner face, an outer face and an edge portion, said frame member having a higher coeflicient of thermal expansion than said window, said frame member having an internal rib adjacent said inner face of said window, said rib turned upwardly to provide a shoulder of smaller diameter than said window portion, the diameter of said frame member being slightly greater than the diameter of said window to provide a region between said frame member and said edge portion of said window, a cushion of filler material having a composition differing from that of said window portion disposed within said region and a layer of like filler material between said rib and said inner face of said window, said inner face of said window being disposed with relation to said rib portion to promote capillary action.

5. A vacuum-tight window for a thermal image or detection device comprising a transmissive window portion of a nonmetallic material sealed to a flexible metallic frame member, said window portion having an inner face, an outer face and an edge portion, said frame member including an internal rib portion, a first annular portion of a first diameter and a second annular portion of a diameter smaller than said first annular portion and integral with said rib portion and said first annular portion, the diameter of said window portion being slightly smaller than the diameter of said second annular portion, said rib portion turned inwardly and upwardly to provide a shoulder of slightly smaller diameter than that of said window, said shoulder being disposed adjacent said inner face of said window and a cushion of filler material having a composition differing from that of said window portion within the region defined by said edge portion and said second annular portion and a capillary thickness of like filler material between said rib portion and said inner face to provide a vacuum-tight seal.

6. A vacuum-tight window structure comprising a radiation transmissive window portion, a frame member sealed to said window portion, said frame member comprising a sleeve portion disposed around said window portion and a rib portion extending from said sleeve portion to form a shoulder portion on which said window portion rests and a channel region between said sleeve portion and said shoulder portion.

7. A vacuum-tight window structure comprising a radiation transmissive window portion of a non-metallic material, said window portion having a par of opposing; faces and an edge, a flexible frame member having a.

coefficient of thermal expansion greater than that of said window portion and comprising a sleeve portion positioned around said edge of said window portion and a'rib portion extending inwardly from said sleeve portion to form a shoulder portion to which one of said faces of said window portion is sealed and a channel region between said sleeve portion and said shoulder portion. i

8. A vacuum-tight window structure for a device sensitive to infrared radiation comprising an infrared radiation transmissive window portion of non-metallic material having a pair of opposing faces and an edge surface, a flexible frame member having a coefiicient of thermal expansion greater than that of said window portion comprising a sleeve portion loosely fitted around said edge surface of said window portion and cushioned therefrom by a filler material of composition differing from that of said window portion and a rib portion extending inwardly from said sleeve portion to form a shoulder portion and a channel region between said sleeve portion and said shoulder portion, said shoulder portion vacuum sealed by a like filler material to one of said faces of said window portion close to said edge surface.

9. A vacuum-tight window structure for a device sensitive to infrared radiation comprising an infrared radiation transmissive window portion of' barium fluoride having a pair of opposing faces and an edge surface disposed approximately perpendicularly to said faces, an

annular, flexible frame member of a noble metal comprising a generally cylindrical sleeve portion loosely fitted around said edge surface of said window portion and cushioned therefrom by a filler material of silver chloride and a rib portion extending inwardly from said sleeve portion to form a shoulder portion and a channel region between said sleeve portion and said shoulder portion, said shoulder portion vacuum sealed by a like filler material to one of said faces of said window portion close to said edge surface.

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